The present invention relates to a method and apparatus for generating pulsed laser radiation of two or more wavelengths, and in particular to a method and apparatus for generating multiple wavelength pulsed laser radiation in a single volume of a laser medium, each multi-wavelength pulse of radiation being emitted in synchronism and on a common axis so that the different wavelengths of the radiation overlap spatially along the propagation path.
Pulsed lasers with multi-wavelength emission are required, for example, for multi-photon excitation of gases for the purpose of isotope separation, for determining contaminant concentrations by the LIDAR (Light Detection And Ranging) method and for multi-wavelength interferometry. For these methods, it is desireable that the pulses of selected wavelengths propagate in a common beam over long paths, that is, that the pulses precisely overlap each other, are in precise synchronism with each other and have a high pulse peak power.
Methods for wavelength tuned generation of pulsed multi-wavelength laser radiation are described in applicable literature. For example, the following references disclose two different known procedures for wavelength tuned generation of pulsed multi-wavelength laser radiation: "Enhancement of the Selectivity and Yield in Infrared Multiphoton Dissociation of Molecules in Multi-Frequency Infrared Laser Fields," by A. V. Evseev et al., Sov. J. Quantum Electron. 15. No. 5, (1985), pages 689-691; "Two-Color TEA-CO.sub.2 Laser Oscillation on the Lines of Regular and Hot Bands," by V. V. Churakov et al., Applied Physics, Vol. 42, (1987), pages 245-249, and "Theoretical and Experimental Studies of a Multiline TEA-CO.sub.2 Laser with Hot CO.sub.2 as an Intracavity Absorber," by A. K. Nath and U. K. Chatterjee, IEEE J. Quant. Electronics QE-16, No. 11, (1980) pages 1263-1266. In these references, laser radiation is generated with the use of a TEA [transversely excited atmospheric pressure] CO.sub.2 laser discharge. The laser medium is under pressure so that those energy exchanges between rotation levels, which play a significant role in the ability of CO.sub.2 laser gas to emit radiation at different wavelengths, take place quickly. Consequently, when one laser line begins to oscillate, occupation of all upper laser levels decreases and occupation of all lower levels increases. Hence, inversion is reduced for all laser transitions as soon as one laser line is emitted. Under such conditions, different wavelengths compete for the same inversion and multi-wavelength emission can either no longer be obtained or is unreliable because the radiation under the more favorable oscillation conditions immediately increases at the expense of the weaker radiation and prevents its formation.
In order to overcome this difficulty, Evseev et al. configure their laser resonator so that radiation components at different wavelengths are extracted from spatially separated partial volumes of the excitation volume. However, this approach has grave drawbacks. In particular, in the case of non-adjacent lines, the optical axes of the partial volumes do not lie closely adjacent one another and consequently only a fraction of the entire excitation volume is utilized. Moreover, radiation at widely spaced wavelengths requires an excitation volume with a large discharge cross section and such large cross sections are difficult to realize. Also, although Churakov et al. and Nath and Chatterjee use an excitation volume which lies in a resonator arm which is common to all wavelengths, the resulting emission is limited to precisely two wavelength combinations or limited to wavelength combinations which cannot be selected independently of one another. In both cases the resonator quality is tuned so that the product of amplification and resonator loss is identical (to the extent that no particular wavelength grows at the expense of another) for all emitted wavelengths. But as already mentioned, such multi-wavelength emission is unreliable because competition between the different wavelengths .lambda..sub.i is not avoided.
More recently, Q switched low pressure (pressure of the laser gas p&lt;100 hecto-Pascal [hPa]) continuous wave lasers rather than TEA lasers (p&gt;&gt;100 hPa) have been developed and successfully employed to produce pulsed CO.sub.2 laser radiation. For example, Q switched low pressure continuous discharge lasers have been developed in order to examine separation of carbon isotopes as discussed in "New Developments in High-Power CW Discharge MultiKilohertz Repetition Rate CO.sub.2 Lasers," by C. D'Ambrosio et al., SPIE, Vol. 1031, GCL-Seventh International Symposium on Gas Flow and Chemical Lasers, Vienna, August, 1988, pages 48-55. Such lasers are intended to be more reliable than TEA lasers since they avoid the susceptible pulsed high voltage discharge required by TEA lasers and revert instead to the mature technology of industrial lasers used in material processing. Evseev et al. show that the use of multi-wavelength laser radiation is advantageous for examining isotope separation of .sup.13 C. Therefore, generation of multi-wavelength radiation with Q switched low pressure lasers also appears to be desirable.